1. Field of the Invention
This invention relates to biosensor strips and methods for preparing biosensor strips.
2. Discussion of the Art
An electrochemical cell is a device comprising a working electrode and a counter electrode, which electrodes are connected to one another electrically. When in use, electrochemical reactions occurring at each of the electrodes cause electrons to flow to and from the electrodes, thus generating a current. An electrochemical cell can be set up either to harness the electrical current produced, for example in the form of a battery, or to detect electrochemical reactions which are induced by an applied current or voltage.
A biosensor is a type of electrochemical cell, in which the electrode arrangement comprises a working electrode, a reference electrode, and a counter electrode (or in place of the reference electrode and counter electrode, an electrode that functions as both reference electrode and counter electrode). Reagents, e.g., enzyme and mediator, that are required for generating a measurable signal upon electrochemical reaction with an analyte in a sample to be assayed, are placed over the working electrode so that the reagents cover at least a portion of the surface of the working electrode.
In other cases, the biosensor includes a reference electrode comprising, for example, a mixture of silver and silver chloride. The reagents are placed over at least the working electrode. However, placing the reagents over the reference electrode will not influence the electrochemical measurement at the working electrode. For example, a reagent containing a quinone mediator would not react with the silver/silver chloride mixture. A biosensor having this type of mediator makes it possible for reagents to be applied over the working electrode with inaccurate registration of the reagent relative to the working electrode.
In still other instances, the reagents of the biosensor are required to be isolated from substances applied to the reference electrode in order to prevent interaction between the mediator and the substances applied to the reference electrode. In these cases, precise registration of the reagents on the working electrode may be required.
The differences between the various types of biosensors are dependent upon the chemical reaction desired. One of ordinary skill in the art can readily modify a given biosensor so as to render it capable of performing the desired chemical reaction.
U.S. Pat. No. 6,863,800, incorporated herein by reference, shows a biosensor strip 10 that contains an electrode arrangement that is suitable for use in this invention. Referring to FIG. 1 of U.S. Pat. No. 6,863,800, an electrode support 11, preferably an elongated strip of polymeric material (e.g., polyvinyl chloride, polycarbonate, polyester, or the like) supports three tracks 12a, 12b, and 12c of electrically conductive ink, preferably comprising carbon. These tracks 12a, 12b, and 12c determine the positions of electrical contacts 14a, 14b, and 14c, a reference electrode 16, a working electrode 18, and a counter electrode 20. The electrical contacts 14a, 14b, and 14c are insertable into an appropriate measurement device (not shown).
Each of the elongated portions of the conductive tracks 12a, 12b, and 12c can optionally be overlaid with a track 22a, 22b, and 22c of conductive material, preferably made of a mixture comprising silver particles and silver chloride particles. The enlarged exposed area of track 22b overlies the reference electrode 16. A layer of a hydrophobic electrically insulating material 24 further overlies the tracks 22a, 22b, and 22c. The positions of the reference electrode 16, the working electrode 18, the counter electrode 20, and the electrical contacts 14a, 14b, and 14c are not covered by the layer of hydrophobic electrically insulating material 24. This layer of hydrophobic electrically insulating material 24 serves to prevent short circuits. The layer of hydrophobic electrically insulating material 24 has an opening 26 formed therein. This opening 26 provides the boundary for the reaction zone of the biosensor strip 10. Because this layer of insulating material is hydrophobic, it can cause the sample to be restricted to the portions of the electrodes in the reaction zone. The working electrode 18 comprises a layer of a non-reactive electrically conductive material on which is deposited a layer 28 containing a working ink for carrying out an oxidation-reduction reaction. At least one layer of mesh 30 overlies the electrodes. This mesh layer 30 protects the printed components from physical damage. The mesh layer 30 also helps the sample to wet the electrodes by reducing the surface tension of the sample, thereby allowing it to spread evenly over the electrodes. A cover 32 encloses the surfaces of the electrodes that are not in contact with the electrode support 11. This cover 32 is a liquid impermeable membrane. The cover 32 includes a small aperture 34 to allow access of the applied sample to the underlying mesh layer 30. The biosensor strip of FIG. 1 is a top-fill biosensor strip, in which the sample wicks to the electrodes via a layer of mesh. FIG. 2 of U.S. Pat. No. 6,863,800 shows an end-fill biosensor strip that does not have a mesh layer. The sample reaches the electrodes via capillary attraction. The biosensor strip 10′ of FIG. 2 employs a cover layer 40 and a spacer layer 42, e.g., a layer of adhesive, between the electrode support 11 and the cover layer 40. The adhesive can be a pressure-sensitive adhesive. The cover layer 40 does not have an aperture. The spacer layer 42 has a slot 44 that provides the boundary of the reaction zone. The liquid sample enters the biosensor strip 10′ via an opening 46 formed at one end of the slot 44 at one end of the biosensor strip 10′. The liquid sample is introduced at the opening 46 and reaches and traverses the reaction zone by means of the action of capillary force.
Application of the cover 32 to the layer of insulating material 24 is currently achieved by aligning the cover above the remaining components to be processed and then clamping the cover 32 to the aforementioned remaining components together with a flat platen and a profiled block. The flat platen is placed below the electrode support 11 and the profiled block is placed above the cover 32. The profiled block is heated prior to the step of laminating the cover 32 to the remaining components of the biosensor strip. FIG. 1 herein shows how the flat platen “P” and the heated, profiled block “B” are aligned during the laminating step. Ensuring that the profiled block “B” is properly aligned with the platen “P” is essential for the success of the process. Proper alignment requires a relatively high degree of skill and considerable time to achieve the bond required for preparing the biosensor strip “S”. Although this method produces an excellent bond, it may also cause a dome to form in the tape between the points where the portions of the profiled block contact the cover 32. The formation of this dome increases the volume of the electrochemical cell unnecessarily. FIGS. 2A, 2B, and 2C herein illustrate graphically how dome cross sections, dome radii, and dome heights of singulated biosensor strips prepared with the flat platen “P” and heated profiled block “B” that is currently used to prepare biosensor strips vary as a function of width of the sample flow channel.
In addition, the method of application currently used to prepare the biosensor strip is an intermittent process, i.e., lamination of the cover to the remaining layers is not carried out continuously. Accordingly, the method of lamination currently used requires the components to be laminated to be indexed to the proper position, have their motion halted at precisely the proper moment, clamped together, and then held together for a specified period of time as the heat transfers from the profiled block, through the backing of the cover and into the layer of adhesive. The clamp then has to be released and the product moved out of the way. Furthermore, reactivating or softening the adhesive while the layer of tape is clamped onto the remaining layers brings about the transfer of a great deal of heat into the remaining layers. Because enzymes are denatured at elevated temperatures, a high level of heat transfer is not desirable.
Reduction in the volume of the electrochemical cell by removing the dome caused by the process employing the platen and profiled block used to adhere the cover to the remaining components of the biosensor strip can be brought about by using a low-profile tape for preparing the cover. A low-profile tape can reduce the volume of the sample flow channel by 33%. The need to reduce the volume of the sample flow channel is driven by the perception that if a lower quantity of blood is required to carry out a test, then a lower amount of pain is experienced by the patient to obtain the required quantity of blood. Previous trials of low-profile tapes that use pressure-sensitive adhesive (PSA) have been known to fail when the cards on which a plurality of the biosensors are printed are converted into individual biosensor strips. The PSA builds up on the cutters of the converting machines, e.g., a packaging machine commercially available from Romaco Siebler and having the tradename “SIEBLER”. This buildup results in the adhesive's falling in lumps into the packaging of the biosensor strips and also requires extensive cleaning of the cutter blades and undercarriage of the converting machine.
It is also known that sample flow channels in biosensor strips need means for air to escape as liquid displaces it. In many products, these means are provided by a single vent opening (see reference numeral 34), in either the upper or lower surface of the biosensor strip, which means that the single vent opening requires proper registration in two directions to provide a reproducible and reliable biosensor strip. In other words, if the vent opening is misaligned in a direction perpendicular to the direction of sample flow, liquid will not enter the sample flow channel; if the vent opening is misaligned in a direction parallel to the direction of sample flow but is still in register with the sample flow channel, liquid will enter the sample flow channel, but the quantity of sample may be insufficient to trigger the assay or perform the assay correctly; if the vent opening is misaligned in a direction parallel to the direction of sample flow but is not in register with the sample flow channel, liquid will not enter the sample flow channel.
As indicated previously, the cover can be adhered into place by a method employing a platen and a profiled block. As also indicated previously, this method creates a dome, which is open to the surrounding environment at the distal end of the sample flow channel. This opening provides a natural vent, but increases the volume of sample required to fill the sample flow channel. The low-profile tapes often bond so well that no air can escape from the sample flow channel, and, consequently, the sample will not flow into the sample flow channel. Forming an opening in the distal end of the sample flow channel allows the air to escape from the sample flow channel and the sample to enter the sample flow channel. Forming an opening in the distal end of the sample flow channel would also aid the flow of a sample in the sample flow channel wherein flow is driven by capillary attraction (see FIG. 2 of U.S. Pat. No. 6,863,800) or by wicking along a layer of mesh, e.g., chemically assisted wicking.
Forming various vents in the sides of the sample flow channel has been attempted, but all such vents result in an unsightly mess as the liquid sample wicks along the vent. Vents formed by perforation techniques comprise one opening in the cover of the biosensor strip. The liquid sample does not wick into the opening formed in the cover. However, as stated previously, a vent formed in the cover requires proper registration in two directions.
The problem of variability of fill rate from biosensor strip to biosensor strip is believed to be caused by adhesive flow and the use of ever finer meshes, thereby resulting in a seal being formed between the cover and the layer of insulating material. The use of fine meshes reduces the quantity of liquid sample, e.g., blood, needed to perform an assay. However, the use of fine meshes also results in a smoother surface in the insulating layer. The method currently used for preparing biosensor strips, i.e., laminating by means of the flat platen and profiled block, encourages the sample flow channel to seal if too much adhesive flows during the lamination process. The degree of sealing directly affects the rate at which a liquid sample fills the sample flow channel. A reliable and reproducible vent is required to ensure minimal variation in fill rate.
In view of the foregoing, it is desired to develop a biosensor strip having a low profile in order to reduce the volume of liquid sample required to perform an assay. It is further desired to develop a means for venting such a low-profile biosensor strip. It is further desired to develop a method for preparing such a biosensor strip in a continuous manner. It is still further desired that this biosensor strip be reproducible and reliable with respect to filling with liquid sample.